Implantable drug infusion device having a flow regulator

ABSTRACT

An implantable drug infusion device which features an improved flow regulator which permits the flow rate to be independent of reservoir pressure within a given pressure range. The flow regulator features a membrane having a hole, the membrane itself positioned above a bottom layer such that sufficient deflection of the membrane causes the membrane to engage against the bottom layer. As liquid flows through the hole a drag force is applied to the edge of the hole resulting in a deflection of the membrane. Once contact is made between the membrane and the bottom layer, then flow reduced. In a further embodiment the bottom layer features a variable flow channel such that upon membrane deflection flow may only proceed through the hole and through the flow channel. By tailoring the shape and length of the variable flow channel the flow characteristics of the regulator versus pressure may be adjusted. In a further embodiment the flow regulator also features a flow sensor integrated therewith. This integrated sensor provides a measurement of flow and may be coupled to the flow regulator to provide feedback thereto.

RELATED APPLICATIONS

This application is related to one or more of the following patentapplication, each of which is hereby incorporated herein by referenceand assigned to the assignee of the present application:

U.S. patent application Ser. No. 09/017,198 filed Feb. 2, 1998 for“System For Locating Implantable Medical Device” to Markus Haller andKoen Weijand;

U.S. patent application Ser. No. 09/017,195 filed Feb. 2, 1998“Implantable Drug Infusion Device Having A Safety Valve” to MarkusHaller and Koen Weijand; and

U.S. patent application Ser. No. 09/017,196 filed Feb. 2, 1998 for“Implantable Drug Infusion Device Having An Improved Valve” to MarkusHaller, T. S. J. Lammerink and Niels Olij.

FIELD OF THE INVENTION

The present invention relates to the field of implantable drug infusiondevices and more particularly to an implantable drug infusion devicehaving a flow regulator.

BACKGROUND OF THE INVENTION

Implantable drug infusion devices are used to provide patients with aconstant or programmable long term dosage or infusion of a drug or anyother therapeutic agent. Essentially such device may be categorized aseither active or passive.

Active drug or programmable infusion devices feature a pump or ametering system to deliver the drug into the patient's system. Anexample of such an active drug infusion device currently available isthe Medtronic SynchroMed™ programmable pump. Such pumps typicallyinclude a drug reservoir, a peristaltic pump to pump out the drug fromthe reservoir, and a catheter port to transport the pumped out drug fromthe reservoir via the pump to a patient's anatomy. Such devices alsotypically include a battery to power the pump as well as an electronicmodule to control the flow rate of the pump. The Medtronic SynchroMed™pump further includes an antenna to permit the remote programming of thepump. Needless to say, in view of these various components, the cost aswell as the size of active drug infusion devices is greater thandesired.

Passive drug infusion devices, in contrast, do not feature a pump, butrather rely upon a pressurized drug reservoir to deliver the drug. Thussuch devices tend to be both smaller as well as cheaper as compared toactive devices. An example of such a device includes the MedtronicIsoMed™. This device delivers the drug into the patient through theforce provided by a pressurized reservoir. In particular, this reservoiris pressurized with a drug to between 20 to 40 psi (1.3 to 2.5 bar) andis used to deliver the drug into the patient's system. Typically theflow path of the drug from the reservoir to the patient includes a flowrestrictor, which permits a constant flow rate. The flow rate, however,is only constant, if the pressure difference between reservoir andpatient does not change. Factors that could impact this pressuredifference include temperature, pressure-volume dependence of reservoirand altitude, among others. The selected pressure for the reservoir isthus typically quite high, so that absolute pressure changes only causesmall and acceptable errors in flow rate. This also requires, however,the drug to be injected into the reservoir using still higher pressure.This is often a very difficult to achieve using a hand operated syringe.

In addition such devices present challenges to accurately deliver aprecise dosage of drug to the patient. As the amount of drug is removedfrom the reservoir, the pressure in the reservoir drops. This, in turn,affects the flow rate such that only over a limited pressure range willthe flow rate be constant. Still further, because the ambient pressurechanges in which the patient exists (due to weather or altitude forexample) the resistance to drug infusion likewise changes, furtheraffecting the flow rate. Temperature will also have a similar impact.

Thus there is a need for a drug infusion system which will permit thedrug flow rate to be independent of reservoir pressure within a givenpressure range.

SUMMARY OF THE INVENTION

The present invention provides an implantable drug infusion device whichfeatures an improved flow regulator which permits the flow rate to beindependent of reservoir pressure within a given pressure range. Theflow regulator features a membrane having a hole, the membrane itselfpositioned above a bottom layer such that sufficient deflection of themembrane causes the membrane to engage against the bottom layer. Asliquid flows through the hole a force is applied to the membrane,resulting in a deflection of the membrane which, in turn, impedes theflow path. In a further embodiment the bottom layer features a variableflow channel such that upon membrane deflection flow may only proceedthrough the hole and through the flow channel. By tailoring the shapeand length of the variable flow channel the flow characteristics of theregulator versus pressure may be adjusted. In a further embodiment theflow regulator also features a flow sensor integrated therewith. Thisintegrated sensor provides a measurement of flow and may be coupled tothe flow regulator to provide feedback thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an implantable drug infusion deviceaccording to the present invention.

FIG. 2 is a side view of a flow regulator according to the presentinvention in which the system pressure is low and the regulator membraneis not deflected.

FIG. 3 is a side view of a flow regulator according to the presentinvention in which the system pressure is high and the membrane isdeflected.

FIG. 4 is a side view of a further embodiment of a flow regulator.

FIG. 5A is a top view of the variable flow restrictor channel used inthe embodiment depicted in FIG. 4 of the present invention.

FIG. 5B is a sectional view of the variable flow restrictor shown inFIG. 5A.

FIG. 5C is a sectional view of an alternative variable flow restrictorchannel.

FIG. 6 depicts the flow versus pressure for one embodiment of thepresent invention showing, in particular, the linear flow between thetwo pressures which may be permitted using this present invention.

FIG. 7 is a block diagram of an implantable drug infusion device whichfeatures an integrated self-test mechanism on the flow regulator.

FIG. 8 is a side view of a flow regulator which features an integratedself-test mechanism on the flow regulator.

FIG. 9 depicts the change in resistance of the piezo-resistors used inthe flow sensors versus reservoir pressure.

FIG. 10 is a flow chart depicting steps employed in a self-test featureaccording to one embodiment of the present invention.

The FIGS. are not necessarily to scale.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an implantable drug infusion device and inparticular of a passive system to deliver drugs and other therapeuticagents. As seen, such a system 1 comprises a reservoir 2, flow regulator3 and outlet catheter 4. The reservoir is a pressurizable reservoir tohold drugs and other therapeutic agents. Reservoir may be of a standarddesign, such as that used in the above mentioned Medtronic IsoMed™implantable drug infusion system. Flow regulator 3 is coupled to thereservoir and the outlet catheter. Flow regulator controls the flow ofmaterial which may be transmitted from the reservoir to the outletcatheter and in particular permits the flow rate to be independent ofreservoir pressure within a given pressure range. System may be refilledthrough injection port 5 through the use of a needle 6 as is well known.Surrounding all components of the implantable pump other than the outletcatheter is a hermetic closure 13 as is well known in the art.

FIG. 2 is a side view of a flow regulator according to the presentinvention. In this view the reservoir pressure is low. As seen, flowregulator comprises a membrane 21, 22 cantilevered from shoulders 23 and24 respectively. In the preferred embodiment membrane is circular,although other shapes may also be used, e.g. rectangular.

Center of the membrane features flow lumen 25. The membrane is furtherdisposed above a substrate 30 such that cavity 31 is defined. Substrate30, in turn, has an outflow tract 32 coupled to cavity 31. Thus, unlessactivated by pressure, the membrane remains in the position as shown andfluid flows through flow lumen 25 into cavity 31 and thereafter throughoutflow tract 32. Outflow tract is coupled, in turn, to outlet catheter(although not shown in this view). Outlet catheter may be of any modeldesired and suited to the patient's requirements.

Depending on the amount of pressure exerted by the fluid, the membranemay be either in the position shown or deflected any amount as permittedby substrate 30. In the preferred embodiment shoulders and membrane aresilicon and substrate is Pyrex™ glass, although other materials may alsobe used such as titanium or tantalum. Moreover, the areas of substrateand membranes in contact with any drug or fluid are further preferablycoated with diamond or diamond-like carbon so as to inhibit anyinteractions between the drug or fluid and the materials. Such coatingsmay be selected according to the particular drug or fluid to be infused.

FIG. 3 is a side view of a flow regulator according to the presentinvention in which the system pressure is high. As seen in thisembodiment, the pressure of the fluid causes the membrane to bedeflected and strike against substrate 30. In such a manner the fluidpathway (flow lumen 25 into cavity 31 and thereafter through outflowtract 32) is blocked by the membrane itself and all fluid flow is thusstopped.

FIG. 4 is an additional embodiment of the present invention and, inparticular, the preferred embodiment of flow regulator which features avariable flow restrictor channel 33. As seen in this embodiment, flowregulator features a variable flow restrictor channel which provides apathway through which flow may continue even though the membrane isdisposed against a surface in substrate 30. In particular, flow proceedsthrough lumen 25 into the variable flow restrictor channel 33 to theoutlet 32. Because membrane strikes the top of substrate all flow isforced to go to the “beginning” of the variable flow restrictor channel.As more pressure is applied to the membrane by the fluid, the membraneis deflected to a greater degree, a greater contact area is made betweenthe membrane and the substrate, and the fluid is forced to flow througha longer pathway through the variable flow restrictor channel. In thepreferred embodiment the length of the flow channel is directlyproportional to the flow resistance. The increase in contact area due topressure proportionally lengthens the distance in which the fluid flowsexclusively within the flow channel. Thus the flow through therestrictor channel is directly proportional to the pressure applied tothe fluid within that channel. This capability thus provides thisembodiment with the ability to directly compensate pressure inaccuraciesas well as pressure variations within any of the system components(upstream of the flow sensor) such as the reservoir, when such pressureanomalies are with the (upstream of the flow sensor) specified pressureregion. Ultimately, this design permits the flow rate to be independentof reservoir pressure within a given pressure range.

FIG. 5A is a top view of a variable flow restrictor channel used in thepreferred embodiment. As seen in this embodiment, restrictor channel isessentially spiral shaped according to the following equation:$x = {{\frac{{a \cdot \cos}\quad t}{t}\quad {and}\quad y} = {{{\frac{{a \cdot \sin}\quad t}{t}\quad {for}}\quad - \infty} < t < {0\quad {and}\quad 0} < t < \infty}}$

where “a” is 1 in the preferred embodiment, although any value betweenapproximately 0.1 to 100 may also be chosen

FIG. 5B is a sectional view of the flow restrictor channel of FIG. 5Ataken along the line 5B—5B. As seen in this embodiment, the restrictorchannel is essentially square in shape and has a depth roughly equal tothe width. Of course, other cross sectional shapes of restrictor channelmay also be used, such as circular, as seen in FIG. 5C or other shapes,triangular, etc. What is important for the flow characteristics of theregulator, however, is the cross sectional area of the channel. In thepreferred embodiment the channel has a width of 15 μm and depth of 10 μmwhich permits a essentially constant flow rate of 500 μl over a pressurerange of between approximately 2 to 8 psi above ambient pressure.Moreover, although the cross sectional area and shape of the restrictorchannel is constant in the preferred embodiment, either the shape orarea or both may be varied along the various portions in order toprovide other flow characteristics besides those of the preferredembodiment.

FIG. 6 is a graph showing the flow rate versus pressure of the preferredembodiment. As seen, due to the usage of the deflected leaflets inconjunction with the variable flow restrictor channel the flow rate maybe caused to be constant over a pressure range. In this chart P1 is 2psi, P2 is 8 psi and F1 is 500 ml.

FIG. 7 is a block diagram of an alternative embodiment of the presentinvention. As seen, such a system 1 comprises a reservoir 2, flowregulator/flow sensor 7, electronic controls 10, battery 11, telemetryassembly 12 and outlet catheter 4. Flow regulator/flow sensor 7 iscoupled to the reservoir across safety valve 16 and further coupled tothe outlet catheter across pump 17. Flow regulator/flow sensor regulatesthe flow of material which may be transmitted from the reservoir to theoutlet catheter by pump in a manner to the flow regulator alreadydescribed above, i.e. it regulates flow such that flow rate isindependent of reservoir pressure within a given pressure range.Moreover, in this embodiment, the flow regulator also functions as aflow sensor to permit the flow rate to be sensed such that the devicecan track how much drug is delivered. Further, this component alsopermits the device to test itself so as to check and monitor the actualflow rate. As already described above, the system may be refilledthrough injection port 5 through the use of a needle 6 as is well known.Surrounding all components of the implantable pump other than the outletcatheter is a hermetic closure 13 as is well known in the art.Electronic controls 10, battery 11, telemetry assembly 12 and pump 17are all constructed in any manner well known in the art. Electroniccontrols are powered by battery 11 and may receive remote operationinstructions via telemetry assembly 12, as is well known in the art.Safety valve is preferably of a design as shown in the co- pendingapplication of Haller et al. “Implantable Infusion Device Having SafetyValve” (P-7356) filed this same day and incorporated herein byreference.

FIG. 8 is a side view of a flow regulator/flow sensor used in the systemof FIG. 7 As seen, this embodiment is essentially the same as that shownin FIG. 4. That is, flow regulator comprises membrane 21 cantileveredfrom shoulders 23 and 24 respectively disposed above a variable flowrestrictor channel within substrate 30. As already discussed above,channel provides a pathway through which flow may continue even thoughthe membrane is disposed against the surface of substrate 30. In thepresent embodiment, the flow regulator/flow sensor further features oneor more piezo-resistive elements 40, 41 integral with the membrane suchthat defoymation or bending of the leaflets is detected by the elements.Such elements are coupled to the electronic controls, which process thesignals and extract information as to element deformation and thus flowthrough the valve. Although piezo-resistive elements are used, othertypes of elements may also be used, such as capacitive or inductive.

FIG. 9 is a graph showing the change in resistance to flow versuspressure of the preferred embodiment. As seen, due to the usage of thedeflected membrane in conjunction with the variable flow restrictorchannel the change is resistance to flow increases in proportion to thepressure.

FIG. 10 is a flow chart depicting the steps used of a self-test featuremade possible through the one or more piezo-resistive elements 40, 41integral with the membrane. In particular this feature is used toquantify membrane deflection. This is important because, the membranesmay, over time, take a set, that is exhibit a permanent deflection. Thusthe self test permits the membrane position to be precisely measured.Such information may be then used to assess device operation, e.g. theactual flow rate of fluid through the regulator. amount of refillreservoir required by the or device malfunction. Typically this selftest procedure is performed at device implant or follow-up by thephysician.

As seen in FIG. 10 at 10-1 a first amount of energy is apply across oneor more piezo-resistive elements 40, 41. Next at 10-2 a parameterindicated through the first amount of energy is sensed. Such parametersmay include resistance, impedance or capacitance, for example. Becausein the preferred embodiment the elements are piezo-resistive, then theparameter preferably sensed would be the electrical resistance in theelements. The exact type of parameter is not crucial to the self testfeature, nor is it whether the elements are piezo resistive or piezocapacitive, etc. Next at 10-3 a second amount of energy is apply acrossone or more piezo-resistive elements 40, 41 while a know pressure isgenerated in the reservoir. Next at 104 a second parameter indicatedthrough the second amount of energy is sensed. At 10-5 the sensed secondparameter is calibrated against the preceding known pressure and thequantity of membrane deflection is determined. This, in turn, indicatesflow. At 10-6 runs a self diagnosis to determines, among other things,whether the sensed flow is within a predetermined range, if not, thenthe device closes a valve and shuts down. Otherwise the device uses thenew data to correct the sensed deflection against the known pressure andcreate a new baseline for future measurements.

Although a specific embodiment of the invention has been disclosed, thisis done for purposes of illustration and is not intended to be limitingwith regard to the scope of the invention. It is contemplated varioussubstitutions, alterations and/or modifications may be made to thedisclosed embodiment without departing from the spirit and scope of theinvention. Such modifications may include substituting elements orcomponents which perform substantially the same function insubstantially the same way to achieve substantially the same result forthose described herein.

What is claimed:
 1. An implantable drug infusion device, comprising: anhermetic enclosure; a fluid reservoir positioned within the hermeticenclosure, the fluid reservoir having means for maintaining a fluidtherein between a first pressure and a second pressure; means fordelivering the fluid into a patient's body; a substrate having a firstfluid outlet port disposed therein, the first fluid outlet port beingoperably connected to the means for delivering a fluid into thepatient's body; a flow regulator comprising a deflectable membranehaving a fluid lumen disposed therein, the flow regulator being coupledto the fluid reservoir, and a fluid pathway contiguous with the firstfluid outlet port and the lumen, at least portions of the fluid pathwaybeing disposed between the first fluid outlet port and the lumen;wherein at least portions of the deflectable membrane, in response tothe pressure of the fluid in the reservoir being between the firstpressure and the second pressure, deflect insufficiently to terminate aflow of the fluid from the reservoir through the lumen and thence thefirst fluid outlet port via the fluid pathway.
 2. An implantable druginfusion device according to claim 1, wherein the means for sensing therate of fluid flow through the flow regulator comprises at least onepiezo-resistive element operably connected to the deflectable membrane.3. An inplantable drug infusion device according to claim 2, furthercomprising means for determining the amount of deflection of themembrane.
 4. An implantable drag infusion device according to claim 1,further comprising means for calibrating the amount of deflection of themembrane in respect of a predetermined pressure of the fluid in thereservoir.